Patients suffering from chronic CHF manifest an elevation of left ventricular end-diastolic pressure, according to the well-known heterometric autoregulation principles espoused by Frank and Starling. This may occur while left ventricular end-diastolic volume remains normal due to a decrease in left ventricular compliance concomitant with increased ventricular wall stiffness. CHF due to chronic hypertension, ischemia, infarct or idiopathic cardiomyopathy is associated with compromised systolic and diastolic function involving decreased atrial and ventricular muscle compliance. These may be conditions associated with chronic disease processes or complications from cardiac surgery with or without specific disease processes. Most heart failure patients do not normally suffer from a defect in the conduction system leading to ventricular bradycardia, but rather suffer from symptoms which may include a general weakening of the contractile function of the cardiac muscle, attendant enlargement thereof, impaired myocardial relaxation and depressed ventricular filling characteristics in the diastolic phase following contraction. Pulmonary edema, shortness of breath, and disruption in systemic blood pressure are associated with acute exacerbations of heart failure.
All these disease processes lead to insufficient cardiac output to sustain mild or moderate levels of exercise and proper function of other body organs, and progressive worsening eventually results in cardiogenic shock, arrhythmias, electromechanical dissociation, and death. In order to monitor the progression of the disease and to assess efficacy of prescribed treatment, it is necessary to obtain accurate measures of the heart geometry, the degree of heart enlargement, and the mechanical pumping capability of the heart, e.g., ejection fraction, under a variety of metabolic conditions the patient is likely to encounter on a daily basis. These parameters are typically measured through the use of external echocardiogram equipment in the clinical setting. However, the measurement procedure is time consuming to perform for even a resting patient and cannot be practically performed replicating a range of metabolic conditions. Typically, the echocardiography procedure is performed infrequently and months or years may lapse between successive tests, resulting in a poor understanding of the progress of the disease or whether or not intervening drug therapies have been efficacious. Quite often, only anecdotal evidence from the patient is available to gauge the efficacy of the prescribed treatment.
Moreover, in many cases, diseased hearts exhibiting LVD and CHF also have conduction defects wherein cardiac depolarizations that naturally occur in one upper or lower heart chamber are not always conducted in a timely fashion either within the heart chamber or to the other upper or lower heart chamber. In such cases, the right and left heart chambers do not contract in optimum synchrony with each other, and cardiac output suffers due to the conduction defects. In addition, spontaneous depolarizations of the left atrium or left ventricle occur at ectopic foci in these left heart chambers, and the natural activation sequence is grossly disturbed. The natural electrical activation system through the heart involves sequential events starting with the sinoatrial (SA) node, and continuing through the atrial conduction pathways of Bachmann's bundle and internodal tracts at the atrial level, followed by the atrio-ventricular (AV) node, Common Bundle of His, right and left bundle branches, and final distribution to the distal myocardial terminals via the Purkinje fiber network. A common type of intra-atrial conduction defect is known as intra-atrial block (IAB), a condition where the atrial activation is delayed in getting from the right atrium to the left atrium. In left bundle branch block (LBBB) and right bundle branch block (RBBB), the activation signals are not conducted in a normal fashion along the right or left bundle branches respectively. Thus, in a patient with LBBB or RBBB, the activation of the ventricles is slowed, and the QRS is seen to widen due to the increased time for the activation to traverse the conduction path. For example, in a patient with LBBB, the delay in the excitation from the RV to the LV can be as high as 120 to 150 ms. Cardiac output deteriorates because the contractions of the right and left heart chambers are not synchronized sufficiently to eject the maximal blood volume. Furthermore, significant conduction disturbances between the right and left atria can result in left atrial flutter or fibrillation.
More particularly, as described in commonly assigned U.S. Pat. No. 6,129,744, patients suffering from LVD are also known to have elevated levels of catecholamines at rest because the body is attempting to increase cardiac output that induce a higher resting heart rate. In addition, the QT interval for such a patient is affected by the catecholamine level and thus has a changed pattern during exercise as well. These patients have a decreased QT response, or smaller change in QT, during exercise, such that the QT interval shortening during exercise is smaller than that found normally. Although QT interval is influenced independently by heart rate alone, as well as by exercise and catecholemines, it is not known to what extent each of these factors or both are responsible for the changed QT response to exercise in LVD patients. However, it is known that patients suffering LVD clearly have a different pattern of QT interval shortening during exercise. Moreover, the changed conductive patterns or a heart in heart failure are manifested by other changes in the PQRST waveforms, particularly an abnormally wide or long duration of the ventricular depolarization signal, or QRS.
These observed conduction defects have caused physicians to prescribe implantation of conventional, atrioventricular (AV) synchronous pacing systems, including DDD and DDDR pacing systems, marketed by Medtronic, Inc. and other companies, in certain patients for treatment of heart failure symptoms. Certain patient groups suffering heart failure symptoms with or without bradycardia tend to do much better hemodynamically with AV synchronous pacing due to the added contribution of atrial contraction to ventricular filling and subsequent contraction. However, fixed or physiologic sensor driven rate responsive pacing in such patients does not always lead to improvement in cardiac output and alleviation of the symptoms attendant to such disease processes because it is difficult to assess the degree of compromise of cardiac output caused by CHF and to determine the pacing parameters that are optimal for maximizing cardiac output, particularly the AV delay.
Determining an optimal AV delay requires performing echocardiography studies or obtaining pressure data involving an extensive patient work-up as set forth in commonly assigned U.S. Pat. No. 5,626,623. Moreover, conventional DDD and DDDR pacemakers pace and sense only in the right atrium and right ventricle and cannot alleviate or alter IAB, LBBB, RBBB and QT interval widening.
Consequently, while some improvement has been reported in certain patients receiving two-chamber DDD or DDDR AV sequential pacemakers, the efficacy of the treatment is not established for larger patient populations. A number of proposals have been advanced for providing pacing therapies to alleviate heart failure conditions and restore synchronous depolarization and contraction of a single heart chamber or right and left, upper and lower, heart chambers as described in detail in the above referenced '744 patent and in commonly assigned U.S. Pat. Nos. 5,403,356, 5,797,970 and 5,902,324 and in U.S. Pat. Nos. 5,720,768 and 5,792,203. The proposals appearing in U.S. Pat. Nos. 3,937,226, 4,088,140, 4,548,203, 4,458,677, 4,332,259 are summarized in U.S. Pat. Nos. 4,928,688 and 5,674,259. The advantages of providing sensing at pace/sense electrodes located in both the right and left heart chambers is addressed in the '688 and '259 patents, as well as in U.S. Pat. Nos. 4,354,497, 5,174,289, 5,267,560, 5,514,161, and 5,584,867.
The medical literature also discloses a number of approaches of providing bi-atrial and/or bi-ventricular pacing as set forth in: Daubert et al., “Permanent Dual Atrium Pacing in Major Intra-atrial Conduction Blocks: A Four Years Experience”, PACE (Vol. 16, Part II, NASPE Abstract 141, p.885, April 1993); Daubert et al., “Permanent Left Ventricular Pacing With Transvenous Leads Inserted Into The Coronary Veins”, PACE (Vol. 21, Part II, pp. 239-245, January 1998); Cazeau et al., “Four Chamber Pacing in Dilated Cardiomyopathy”, PACE (Vol. 17, Part II, pp. 1974-1979, November 1994); and Daubert et al., “Renewal of Permanent Left Atrial Pacing via the Coronary Sinus”, PACE (Vol. 15, Part II, NASPE Abstract 255, p. 572, April 1992).
In most cases, it has been proposed that bi-ventricular pacing pulses be applied simultaneously to the right and left ventricles. An observation is made in commonly assigned U.S. Pat. No. 6,219,579 that the exact timing of mechanical events are important for properly controlling right and left heart chamber pacing so as to optimize left ventricular output. Specifically, it is known that actual contraction of one ventricular chamber before the other has the effect of moving the septum so as to impair full contraction in the later activated chamber. Thus, while concurrent or simultaneous pacing of the left and right ventricle may achieve a significant improvement for CHF patients, it is better to provide for pacing of the two ventricles in such a manner that the actual mechanical contraction of the left ventricle, with the consequent closing of the valve, occurs in a desired time relationship with respect to the mechanical contraction of the right ventricle and closing of the right value. For example, if conduction paths in the left ventricle are impaired, delivering a pacing stimulus to the left ventricle at precisely the same time as to the right ventricle may nonetheless result in left ventricular contraction being slightly delayed with respect to the right ventricular contraction.
In the above-referenced '324 patent, an AV synchronous pacing system is disclosed providing three or four heart chamber pacing through pace/sense electrodes located in or adjacent one or both of the right and left atrial heart chambers and in or adjacent to the right and left ventricular heart chambers. During an AV delay and during a V-A escape interval, a non-refractory ventricular sense event detected at either the right or left ventricular pace/sense electrodes starts a programmable conduction delay window (CDW) timer. A ventricular pace pulse is delivered to the other of the left or right ventricular pace/sense electrodes at the time-out of the CDW if a ventricular sense event is not detected at that site while the CDW times out. However, it is not always easy to determine just how to program the CDW duration to optimize the hemodynamics of the heart. As a consequence, it is important to provide a technique for measurement of mechanical events, such as a mechanical closure point of each of the ventricles, so as to be able to accurately program the sequence of pacing to achieve the desired dual ventricular pacing which optimizes ejection fraction, or cardiac output, for the individual patient.
Moreover, while such AV sequential, three or four-chamber pacing systems can be programmed to at least initially restore right and left and upper and lower heart synchrony in the clinical setting, they are not always able to maintain that synchrony over a range of heart rates and as the patient is exposed to other conditions of daily life including stress and exercise. Therefore, a number of other approaches have been proposed and advanced involving implantation of physiologic cardiac monitors for deriving and storing electrical EGM signals and mechanical performance indicating parameters over a prolonged time period and development of three and four-chamber pacing systems having the same capabilities. For example, the Medtronic® CHRONICLE® IHM implantable heart monitor senses blood pressure within a heart chamber and the EGM of the heart using an EGM and pressure sensing lead of the type disclosed in commonly assigned U.S. Pat. No. 5,564,434. Such implantable monitors when implanted in patients suffering from cardiac arrhythmias or heart failure accumulate date and time stamped data that can be of use in determining the condition of the heart over an extended period of time and while the patient is engaged in daily activities.
It is understood that the amount of blood being pumped by the heart is governed not only by the intrinsic or multi-chamber paced heart rate, but also by the stroke volume of the heart, which is adversely lessened by heart failure. It has been recognized that it would be desirable to measure the contractility or displacement of the heart wall to determine the hemodynamic efficiency of the heart alone in an implanted monitor or in the context of controlling the operations of therapy delivery IMDs.
For example the use an accelerometer positioned within a lead that is located within one of the chambers of the heart is disclosed in U.S. Pat. No. 5,549,650. The lead is attached to one of the walls of the heart so that movement of the wall of the heart causes the accelerometer that to develop an accelerometer signal that is processed to provide a first signal indicative of the contractility of the heart and a second signal indicative of the physical displacement of the wall of the heart. It is proposed in U.S. Pat. No. 4,730,619 to derive a measure of the ejection time of the ventricles, which is derived from the duration of contraction of the right ventricle, which is determined from changes in right ventricular pressure. The right ventricular blood pressure is measured by a hermetically sealed absolute strain gauge transducer or a piezoresistive transducer mounted within a transvenous lead. The signals derived in the '650 and '619 patent are employed by the pacing system to adjust the pacing parameters to improve the hemodynamic efficiency of the heart as this information is directly related to the volume of blood being pumped by the heart during each ventricular contraction.
In an approach related to monitoring rejection of heart transplants, a magnetic field responsive Hall effect device and a permanent magnet are implanted directly across the septum or a heart wall as taught in U.S. Pat. No. 5,161,540, and the Hall effect device is powered by an implantable generator and telemetry transceiver. The compliance of the heart wall is monitored to detect any loss of compliance characteristic of rejection of the heart transplant is transmitted from the implanted system.
A discussion of a wide number of mechanical and electrical parameter sensors employed in the art to assess cardiac functions and hemodynamic efficiency is set forth in U.S. Pat. No. 5,243,976. In the '976 patent, continuous wave (CW) and pulsed wave (PW) Doppler emitters are incorporated into pacing leads to measure blood flow, and the flow measurements are employed to regulate atrial and ventricular pacing parameters and for other purposes.
In the above-referenced '579 patent, impedance measurements are made in or across the heart chambers from which accurate timing signals are obtained reflecting mechanical actions, e.g., valve closures, so that accurate timing information is available for controlling electrical activation and resultant mechanical responses for the respective different heart chambers. The impedance or mechanical sensing determinations are preferably made by multiplexing through fast switching networks to obtain the desired impedance measurements in different heart chambers. In a preferred embodiment, control of left heart pacing, is based primarily upon initial detection of a spontaneous signal in the right atrium, and upon sensing of mechanical contraction of the right and left ventricles. In a heart with normal right heart function, the right mechanical AV delay is monitored to provide the timing between the initial sensing of right atrial activation (P-wave) and right ventricular mechanical contraction. The left heart is controlled to provide pacing which results in left ventricular mechanical contraction in a desired time relation to the right mechanical contraction; e.g., either simultaneous or just preceding the right mechanical contraction; cardiac output is monitored through impedance measurements, and left ventricular pacing is timed to maximize cardiac output. In patients with IAB, the left atrium is paced in advance of spontaneous depolarization, and the left AV delay is adjusted so that the mechanical contractions of the left ventricle are timed for optimized cardiac output from the left ventricle.
A CHF monitor/stimulator is disclosed in commonly assigned U.S. Pat. No. 6,104,949 that senses the trans-thoracic impedance as well as patient posture and provides a record of same to diagnose and assess the degree and progression of CHF. The sensed trans-thoracic impedance is dependent on the blood or fluid content of the lungs and assists in the detection and quantification of pulmonary edema symptomatic of CHF. Trans-thoracic impedance is affected by posture, i.e. whether the subject is lying down or standing up, and the sensed trans-thoracic impedance is correlated to the output of the patient posture detector to make a determination of presence of and the degree of pulmonary edema for therapy delivery and/or physiologic data storage decisions.
A monitor/stimulator is disclosed in U.S. Pat. No. 5,417,717 that monitors and assesses level of cardiac function then permits a physician to arbitrate the therapy mode, if therapy is indicated. The monitor stimulator assesses impedance, EGM, and/or pressure measurements, and then calculates various cardiac parameters. The results of these calculations determine the mode of therapy to be chosen. Therapy may be administered by the device itself or a control signal may be telemetry transmitted to various peripheral devices aimed at enhancing the heart's function. Alternatively, the device may be programmed to monitor and either store or telemeter information without delivering therapy.
Particularly, the implantable monitor/stimulator monitors conventional parameters of cardiac function and contractile state, including all phases of the cardiac cycle. Thus, assessments of contractile state measured include indices of both cardiac relaxation and contraction. Utilizing the dual source ventricular impedance plethysmography technique described in U.S. Pat. No. 4,674,518, the monitor/stimulator monitors cardiac function by assessing hemodynamic changes in ventricular filling and ejection or by calculating isovolumic phase indices by known algorithms. The primary calculations involve: (1) the time rate of change in pressure or volume, dP/dt or dV/dt, as isovolumic indicators of contractility; (2) ejection fraction as an ejection phase index of cardiac function according to the known quotient of stroke volume divided by end diastolic volume; (3) Maximal elastance, EM; (4) regression slope through maximal pressure-volume points as a further ejection phase index of contractility using the method of Sagawa; (5) stroke work according to the known pressure-volume integration; (6) the time course of minimum (end) diastolic pressure-volume measurements according to the method of Glantz as a measure of diastolic function; and (7) cardiac output calculation according to the known product of heart rate and stroke volume as an index of level of global function.
While measurement and storage of this group of parameters of cardiac function and contractile state can provide valuable information about the state of heart failure, the sensors are not always easy to implant so that they perform reliably chronically and under the range of conditions encountered by the patient and resulting from progression of the heart failure.
The proposed systems employing locally disposed accelerometers at one or more location in the heart or distributed impedance measuring electrodes to detect and measure heart motion and to derive the above-described parameters are difficult to implement and subject to outside influences that distort the signals. The accelerometer signal needs to be filtered to reject output signals due to non-cardiac contraction forces transmitted through the body by exercise, respiration and blows to the body. The sensitive semiconductor or miniature beam elements of accelerometers are subject to damage or change in sensitivity over time. The foreign body reaction tissue build-up encasing the accelerometer structure can dampen its response to movement or inhibit its movement.
Implantable pressure sensors that employ a diaphragm that is deflected by contacting blood have proven to be problematic. The foreign body reaction induced tissue encapsulation occurring over the diaphragm can decrease its sensitivity to blood pressure fluctuations.
Impedance measuring electrodes on lead bodies in a heart chamber or cardiac blood vessel also become encased in tissue which may tend to change the impedance that is measured over time. Impedance across the right and left ventricles and interventricular septum is also influenced by breathing and patient activity such that these influences must also be filtered out of the signal.
Momentary changes to a patient's autonomic state can change blood pressure (P), heart rate, and pressure rate of change (dP/dt) contractility measures and not be reflective of a “true” functional state change of the heart. Such momentary changes in autonomic state are caused by postural changes as noted in the above-referenced '949 patent and other movements, such as bending down to pick up an object or suddenly standing up from a sitting or reclining position.
It would be desirable to obtain cardiac data that provides an enhanced assessment of cardiac function and heart failure state that are less sensitive to exercise, breathing, patient movements and posture changes, that is simple, inexpensive, easy to implant, reliable and not prone to loss of sensitivity or drift due to tissue encapsulation.
Given the demonstrated feasibility of four-chamber cardiac pacing, and the availability of techniques for sensing natural cardiac signals and mechanical events, there nonetheless remains a need for developing a system which is adapted to the cardiac condition of a patient with CHF, so as to provide pacing sequences which are tuned for improving cardiac output, and in particular for improving left heart function.